1. Field of the Invention
The present invention relates generally to frequency guiding filters for a dispersion managed soliton transmission system, and more particularly pertains to sliding frequency guiding filters using a wavelength locked feedback loop for a dispersion managed soliton transmission communication system.
The present invention uses wavelength locked feedback loops in frequency guide filters, and particularly in sliding frequency guide filters. The wavelength locked feedback loops offer various advantages over conventional methods. The main benefit is that the wavelength locked feedback loops allow precise control over the location of the filter peak center wavelength with respect to the transmitted signal peak center wavelength, to compensate for factors such as the filter rolloff, signal spectral width, and changes in transmission line properties due to temperature, microbending, aging and other effects. This approach allows the construction of very inexpensive frequency guiding filters, and also provides a new degree of freedom in the guide filter design (the guide filter can be designed with a dynamically controllable offset and sliding range). Taken together, these advantages make it possible to design new types of dispersion managed soliton transmission networks.
2. Discussion of the Prior Art
Long distance fiber optic communication systems have made increasing use of soliton transmissions to avoid dispersion and nonlinear effects that can limit both the distance and the bandwidth (maximum achievable data rate) thereof. FIG. 1a illustrates a fiber optic transmission system which typically consists of a laser source 1 and an optical receiver 2 connected by strands of glass fiber 3. The fiber attenuates optical signals from the transmitter; since the receiver has a limited sensitivity, signals can only be detected beyond a certain signal-to-noise ratio. The fiber also induces dispersion or pulse spreading which further degrades the receiver signal levels. Both attenuation and dispersion increase with distance and are more pronounced at higher data rates, which limits both the distance and data rate of a linear transmission system. One brute force approach to increasing the distance is to launch higher optical power levels into the fiber; beyond a certain point, this induces nonlinear effects which once again limit distance and bandwidth.
However, by inducing a controlled form of nonlinearity, it is possible to create optical pulses called solitons which do not change shape as they propagate. Solitons have become widely used in many long distance telecommunication systems, including dense wavelength division multiplexing (DWDM) systems, and in other data communication systems as well. However, various problems are associated with soliton transmission, in particular timing and frequency jitter.
Timing jitter can result from fluctuations in the frequency components of a soliton optical pulse; this imposes severe limitations on the signal to noise ratio. By controlling the frequency of the solitons, it is possible to control timing jitter as well. One approach to controlling the frequency of solitons is to periodically insert narrowband filters in a fiber optic link, usually at optical amplifier locations; these are known as frequency-guiding filters. If for some reason the center frequency of a soliton is shifted from the filter peak, the filter-induced differential loss across the pulse spectrum adjusts the pulse frequency. As a result, the pulse spectrum returns to the filter peak over some characteristic damping length L. If the damping length is considerably less than the transmission distance, then guiding filters can dramatically reduce the timing jitter. Although even linear fiber optic transmission systems will exhibit similar effects, guiding filters of this type can only be used in soliton based transmission systems. Every time an optical pulse passes through a guiding filter, its spectrum narrows; solitons can quickly recover their bandwidth through the fiber nonlinearity, whereas in a linear transmission system the filter acts to continuously degrade the signal.
Another variation is the sliding frequency gain filter in which the transmission peak of each guiding filter is shifted in frequency with respect to the transmission peak of the previous filter, so that the center frequency slides with distance at a predetermined rate. Because of their nonlinearity, solitons can follow the filters and slide in frequency, while linear noise is suppressed. By introducing sliding frequency guiding filters periodically positioned along the length of the transmission line, shown at 4-1, 4-2 in FIG. 1a, an optical transmission line using solitons becomes effectively an all-optical passive regenerator, compatible with DWDM networks. All nonsoliton components of the signal pulse are absorbed by the filters; while input pulses which are close to the optimal soliton profile are reshaped by the transmission line filters into propagating solitons. Note also that the filters act to remove energy fluctuations from the input optical signal, with a damping length close to the frequency damping length. This also acts to self-equalize the energy of different channels in a WDM transmission system. Feedback from the frequency guiding filters locks the energy of individual soliton channels to values that do not change with distance, even if optical amplifiers in the path have different gains at different wavelengths. Sliding frequency guide filters also reduce the timing and frequency drifts associated with effects such as soliton collisions and four wave mixing; they can also be used to construct hybrid transmission lines containing a mix of optical amps, positive dispersion fiber, negative dispersion fiber, and other optical elements which are designed to counterbalance each other and result in nearly flat average dispersion over long distances. Theoretical performance of 10 Gbit/s signals over 40,000 km or 20 Gbit/s signals over 14,000 km have been suggested using these techniques.
Conventional designs for sliding frequency filters have met with some success, but continue to face performance problems; in particular, etalon filters are not a good approximation of ideal parabolic filters, especially when large frequency excursions of solitons are involved; the curvature of the filter response reduces with the deviation of the frequency from the filter peak.
The following is additional background information about soliton transmission. Optical signals propagating in a glass fiber experience dispersion; an optical pulse of width t has a finite spectral bandwidth 1/t. When the pulse is transform limited, all of the spectral components have the same phase. In the time domain, all of the spectral components overlap in time. Because of dispersion, different spectral components propagate in the fiber with different group velocities; thus as the pulse propagates its frequency components spread out in time. The direction of this spreading, or chirp, depends on the sign of the group velocity dispersion (GVD), either positive or negative. There is also a nonlinear effect, self-phase modulation (SPM) resulting from the interaction between the light intensity and the nonlinear portion of the fiber""s refractive index (also known as the Kerr effect). This produces a frequency shift determined by the time derivative of the pulse shape. In silica based fibers SPM always produces a positive chirp (shifts the leading edge of the pulse to the red spectral region). If both GVD and SPM are applied to an optical pulse with opposite signs, the two effects cancel each other out to yield a pulse which does not change shape as it propagates. The resulting pulse is known as a soliton, and is a nondispersive solution of the nonlinear Schrodinger equation. Pulses injected into a fiber which are close in shape to a soliton will be adjusted by the nonlinear effects to reform into stable soliton pulses.
One source of error in soliton systems is the fluctuation in pulse arrival times, or timing jitter. Spontaneous emission noise added to the optical signal modulates the carrier frequencies of the solitons at random. The chromatic dispersion of the fiber then converts the frequency variations into variations in pulse arrival times; this is known as the Gordon-Haus effect. Data errors occur when a pulse arrives outside its acceptable timing window. Thus this effect limits the maximum data bit rate and transmission distance. Another source of timing jitter is the acoustic interaction of pulses; electrostriction effects in the fiber result in each optical pulse generating an acoustic wave in the fiber. Other pulses experience a refractive index change caused by the acoustic wave. The resulting frequency changes of the pulse lead to timing jitter through the fiber""s chromatic dispersion. Both of these error sources place severe limitations on the propagation distance and bit error rate which can be achieved. By controlling the frequency of the solitons, one can also control the timing jitter. This may be done by inserting narrowband filters periodically in the fiber path (so-called frequency guiding filters), usually at optical amplifier locations. If the center frequency of the soliton is shifted from the filter peak, the filter-induced differential loss across the pulse spectrum pushes the pulse frequencies back to the filter peak within some characteristic damping length, L. As long as L is less than the transmission distance, guiding filters significantly reduce the timing jitter.
The damping properties of guiding filters are determined mainly by the curvature of the filter response in the neighborhood of the filter peak. Thus shallow Fabry-Perot etalon filters can be used as guiding filters. These have multiple peaks, and each peak can be used for a different WDM channel, for example. The ability of guiding filters to control timing jitter is determined by the filter characteristics and the soliton spectral bandwidth.
The explanations herein discuss both wavelength and frequency, which have a reciprocal relationship (xcex=c/f, where c=speed of light), as is well known in the field of optics.
Wavelength Division Multiplexing (WDM) and Dense Wavelength Division Multiplexing (DWDM) are light-wave application technologies that enable multiple wavelengths (colors of light) to be paralleled into the same optical fiber with each wavelength potentially assigned its own data diagnostics. Currently, WDM and DWDM products combine many different data links over a single pair of optical fibers by re-modulating the data onto a set of lasers, which are tuned to a very specific wavelength (within 0.8 nm tolerance, following industry standards). On current products, up to 32 wavelengths of light can be combined over a single fiber link with more wavelengths contemplated for future applications. The wavelengths are combined by passing light through a series of thin film interference filters, which consist of multi-layer coatings on a glass substrate, pigtailed with optical fibers. The filters combine multiple wavelengths into a single fiber path, and also separate them again at the far end of the multiplexed link. Filters may also be used at intermediate points to add or drop wavelength channels from the optical network.
Ideally, a WDM laser would produce a very narrow linewidth spectrum consisting of only a single wavelength, and an ideal filter would have a square bandpass characteristic of about 0.4 nm width, for example, in the frequency domain. In practice, however, every laser has a finite spectral width, which is a Gaussian spread about 1 to 3 nm wide, for example, and all real filters have a Gaussian bandpass function. It is therefore desirable to align the laser center wavelength with the center of the filter passband to facilitate the reduction of crosstalk between wavelengths, since the spacing between WDM wavelengths are so narrow. In commercial systems used today, however, it is very difficult to perform this alignmentxe2x80x94lasers and filters are made by different companies, and it is both difficult and expensive to craft precision tuned optical components. As a result, the systems in use today are far from optimal; optical losses in a WDM filter can be as high as 4 db due to misalignment with the laser center wavelength (the laser""s optical power is lost if it cannot pass through the filter). This has a serious impact on optical link budgets and supported distances, especially since many filters must be cascaded together in series (up to 8 filters in current designs, possibly more in the future). If every filter was operating at its worst case condition (worst loss), it would not be possible to build a practical system. Furthermore, the laser center wavelengths drift with voltage, temperature, and aging over their lifetime, and the filter characteristics may also change with temperature and age. The laser center wavelength and filter bandwidth may also be polarization dependent. This problem places a fundamental limit on the design of future WDM networking systems.
A second, related problem results from the fact that direct current modulation of data onto a semiconductor laser diode causes two effects, which may induce rapid shifts in the center wavelength of the laser immediately after the onset of the laser pulse. These are (1) frequency chirp and (2) relaxation oscillations. Both effects are more pronounced at higher laser output powers and drive voltages, or at higher modulation bit rates. Not only can these effects cause laser center wavelengths to change rapidly and unpredictably, they also cause a broadening of the laser linewidth, which can be a source of loss when interacting with optical filters or may cause optical crosstalk. Avoiding these two effects requires either non-standard, expensive lasers, external modulators (which are lossy and add cost), or driving the laser at less than its maximum power capacity (which reduces the link budget and distance). Lowering the data modulation rate may also help, but is often not an option in multi-gigabit laser links.
It would thus be highly desirable to provide a stable, optimal alignment between a laser center wavelength and the center of a Gaussian bandpass filter in order to optimize power transmission through such fiber optic systems and reduce optical crosstalk interference in optical networks.
Accordingly, it is a primary object of the present invention to provide a frequency guiding filters for dispersion managed soliton transmission communication systems using a wavelength locked feedback loop.
The present invention concerns wavelength selective devices which encompass wavelength selective devices of all types including filters of all types including comb filters, etalon filters and rotatable disc filters and wavelength selective gratings of all types including Bragg gratings and array waveguide gratings.
It is an object of the present invention to provide a servo-control xe2x80x9cwavelength-locked loopxe2x80x9d circuit that enables real time mutual alignment of an electromagnetic signal having a peaked spectrum function including a center wavelength and a wavelength selective device implementing a peaked passband function including a center wavelength, in a system employing electromagnetic waves.
It is another object of the present invention to provide a servo-control system and methodology for WDM and DWDM systems and applications that is designed to optimize power through multi-gigabit laser/optic systems.
It is a further object of the present invention to provide a wavelength-locked loop for an optical system that enables real time alignment and tracking of any spectral device that selects a wavelength, such as a Bragg grating, in optical fibers and waveguides, etc., for use in WDM systems.
It is yet another object of the present invention to provide a servo/feedback loop for an optical system, referred to as a xe2x80x9cwavelength-locked loop,xe2x80x9d that enables real time alignment of a laser with variable optical attenuators by offsetting an optical filter from a known transmission in optical fibers and waveguides, etc.
It is yet a further object of the present invention to provide a servo/feedback loop for an optical system, referred to as a xe2x80x9cwavelength-locked loop,xe2x80x9d that may be used in light polarization applications.
It is still another object of the present invention to provide a servo/feedback loop for an optical system, referred to as a xe2x80x9cwavelength-locked loop,xe2x80x9d that enables real time alignment and tracking of laser center wavelengths and filter passband center wavelengths in multi-gigabit laser/optical systems such that the optical loss of a WDM filter/laser combination is greatly reduced, thereby enabling significantly larger link budgets and longer supported distances.
It is yet still another object of the present invention to provide a servo/feedback loop for an optical system, referred to as a xe2x80x9cwavelength-locked loop,xe2x80x9d that enables real time alignment and tracking of laser center wavelengths and filter passband center wavelengths in multi-gigabit laser/optical systems such that lower cost lasers and filters may be used providing a significant cost reduction in the WDM equipment.
When employed in laser/optical networks, the system and method of the present invention may be used to tune for any type of wavelength-selective element in the network, including wavelength selective filters, attenuators, and switches, in fiber Bragg gratings, ring resonators in optical amplifiers, external modulators such as acousto-optic tunable filters, or array waveguide gratings. This applies to many other optical components in the network as well (for example, optical amplifiers that may act as filters when operating in the nonlinear regime). Furthermore, the system and method of the invention may be used to implement less expensive devices for all of the above application areas.
Alternately, the system and method of the invention may be implemented to tune such devices for WDM and optical network applications, in real-time, during manufacture. This would significantly increase lot yields of such devices which otherwise may be discarded as not meeting wavelength specifications as a result of manufacture process variations, for example.
The wavelength locked loop of the present invention enables a tighter control of wavelength, which allows an increased density of wavelength channels with less cross talk between channels in a wavelength multiplex system, which might typically include 32 or 64 channels or links. Pursuant to the present invention, each channel includes a separate wavelength locked loop which includes a separate source such as a laser and wavelength selective device such as a filter. Accordingly a wavelength multiplex system can include an array of 32 or 64 lasers and an array of 32 or 64 filters.